The present invention relates to a circuit arrangement, which is provided for the analogue suppression of echoes as regards the received signal of a communication device connected via a transformer to a transmission line.
In the case of ISDN and DSL transmission systems, data is transmitted between the exchange and the subscriber via a twisted line wire pair, whereby each of the two line wires is provided simultaneously for both transmission directions, that is to say, for the transmission direction from the exchange to the subscriber and for the transmission direction from the subscriber to the exchange. The signal lying on the line wires is, therefore, composed of a received signal portion and a transmitted signal portion. In order to obtain the received signal—also described as “far end signal”—on the subscriber end, the transmitted or echo signal—also described as “near end echo”—must be subtracted from the overall signal. On the one hand, this can be achieved by adaptive echo canceling, which is implemented digitally. On the other hand, this can be achieved by a so-called hybrid-circuit, which simultaneously executes two-wire/four-wire conversion, in order to separate the transmitted and received signal. Hybrid-circuits are, for example, used in telecommunication transmitting apparatus. Possible configurations of known hybrid-circuits are, for example, described in “Telephone Voice Transmission”, Winston D. Gayler, Prentice Hall, 1989.
Echo suppression by a hybrid-circuit has two important advantages. On the one hand, the ratio between the received signal (“far end signal”) and the transmitted or echo signal (“near end echo”) is boosted by the degree of echo suppression, typically by approx. 20 dB. As a result, less strict requirements on the signal to noise ratio of the analogue/digital converter provided for further processing of the received signal are necessary. On the other hand, in conventional ISDN- or xDSL-transmission systems, a digital linear echo compensator is coupled after the analogue part of the received signal path. This digital linear echo compensator is not able to compensate non-linear distortions of the transmitted signal path, which are found in the echo. Therefore, the ratio between the non-linear distortions and the actual signal portion in the received signal path must be kept very low. The echo suppression of a hybrid-circuit can support this, since the portion of the transmitted signal in the received signal and, therefore, the non-linear distortions, are reduced.
The main technical problem, which arises in the case of hybrid-circuits, is the precise emulation of the transmitted signal related to that point in the transmission system, at which the emulated transmitted signal should be subtracted from the overall signal. The transmitted voltage occurring at a particular point of the transmission system is, essentially, a function of the impedance valid at this point of the transmission line as well as of the transformer provided for transmission. This impedance is greatly variable, since lines of different material and length are used in the application with or without so-called “bridge taps”. The term “bridge tap” describes stub cables connected to the line wires of the transmission line, which are provided for the connection of further subscribers, but which are not terminated with a suitable intrinsic impedance and can cause reflections as a result. The problem, therefore, arises of emulating as accurately as possible the impedance of the transmission line and transformer in all cases of application. In the case of an SDSL transmission system (“symmetric digital subscriber line”), such impedance simulation must be optimised in the frequency band of 0 to 400 kHz.
A further system-dependent requirement on the hybrid-circuit in an SDSL transmission system is a signal to noise ratio of at least 90 dB. Further, the power loss of the hybrid-circuit should be as low as possible.
Various circuit arrangements for the suppression of analogue echoes are known from the state of the art.
Thus, for example, in “A CMOS analogue front-end IC for DMT ADSL”, C. Conroy et al., 1999 IEEE International Solid-State Circuits Conference, ISSCC 99, Session 14, Paper TP 14.2, it is proposed to implement two identical transmitted signal paths, whereby the first transmitted signal path is used for the primary transmitted signal and the second transmitted signal path to emulate the echo voltage, in order to be able to supply these subsequently on systems with analogue echo suppression.
In “An Integrated Adaptive Analogue Balancing Hybrid for Use in ADSL Modems”, F. Pécourt et al., 1999 IEEE International Solid-State Circuits Conference, ISSCC 99, Session 14, Paper TP 14, it is proposed to emulate the transmitted signal appearing in each case at the interesting point of the transmission line as a result of the fact that the transmitted signal produced by the line driver of the particular communication device is filtered by means of an integrated active filter. The filter produces an emulation of the echo occurring in the received signal path, so that echo suppression as regards the received signal can be achieved through subsequent subtraction of the output signal of the filter from the received signal of the corresponding hybrid-circuit.
Finally, a generic circuit arrangement for the suppression of analogue echoes is known from “A 25 kft 768 Kb/s CMOS Transceiver for Multiple Bit-Rate DSL”, M. Moyal et al., 1999 IEEE International Solid-State Circuits Conference, ISSCC 99, Session 14, Paper TP 14.4. In this document, a hybrid-circuit is proposed, in which the transmission line is emulated by a scaled impedance model, a so-called replica. Parts of this replica are constructed outside the chip. Additionally, in this document, it is proposed to emulate the main and scatter inductance of the transformer inside the chip by scaled inductive resistors, whereby these inductive resistors are implemented by so-called gyrators. In general, gyrators are understood to mean active circuits with, for example, operation amplifiers and capacitors, which simulate these without using an inductive resistor. Bridge taps are emulated by a RLC-network (“replica”) inside the chip, whereby the inductive resistor is also implemented here by a gyrator circuit.
The previously described solutions for emulating the transmitted signal, related to that point in the transmission system, at which subtraction from the received signal should take place, have various disadvantages. In the case of the first solution described above, considerable technological circuit complexity is necessary, since the overall transmitted signal path must be implemented twice. Any mismatch between the two transmitted signal paths results in inadequate echo suppression. In the case of the second solution previously described, the active filter provided to filter the transmitted signal is active over the overall frequency band. Considerable power consumption is required to keep noise within tolerable limits. In the case of the third solution previously described, on the other hand, the gyrators are active over the overall frequency band. Very high power consumption is also required in this case to keep noise within tolerable limits.